1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to methods and systems for mobile station handoff.
2. Description of Related Art
Wireless telecommunications networks typically include a plurality of distinct wireless coverage areas, e.g., “cells” or “sectors,” within which mobile stations can wirelessly communicate with the network. A base transceiver station (BTS) may provide a single wireless coverage area, e.g., a cell. Alternatively, a BTS may provide multiple wireless coverage areas, e.g., by using directional antennas to provide multiple sectors within the cell and/or by using frequencies. Each wireless coverage area may have allocated to it a distinct set of downlink channels that the BTS uses for transmitting signals to mobile stations and a distinct set of uplink channels that mobile stations may use for transmitting signals to the BTS. The downlink channels may include, for example, a pilot channel, paging channels, and traffic channels. The uplink channels may include, for example, access channels and traffic channels. A mobile station may be associated with a particular wireless coverage area by using or monitoring one or more of the wireless coverage area's channels. For example, a mobile station that is engaged in a call may be associated with a particular wireless coverage area by using uplink and downlink traffic channels allocated to that particular wireless coverage area. Even if not engaged in a call, a mobile station may be associated with a particular wireless coverage area, for example, by monitoring a paging channel allocated to that particular wireless coverage area.
A mobile station's associated wireless coverage area may change, for example, when the mobile station changes its location. Thus, when a mobile station is associated with a first or “source” wireless coverage area, a “handoff” process may be used so that the mobile station becomes associated with a second or “target” wireless coverage area. A handoff may occur when the mobile station is in an active state, e.g., when the mobile station is engaged in a call. For example, before the handoff, the mobile station may use traffic channels allocated to a first wireless coverage area, and, after the handoff, the mobile station may use traffic channels allocated to a second wireless coverage area. A handoff may also occur when the mobile station is an idle state. For example, before the handoff, the mobile station may monitor a paging channel allocated to a first wireless coverage area, and, after the handoff, the mobile station may monitor a paging channel allocated to a second wireless coverage area.
The decision whether to effect a handoff of a mobile station may be based, at least in part, on signal strength measurements taken by the mobile station. For example, the mobile station may measure the signal strength of the pilot channel of the wireless coverage area with which the mobile station is currently associated and the signal strengths of the pilot channels of one or more “neighboring” wireless coverage areas. A handoff controller may send the mobile station a list, e.g., a “neighbor list” or “extended neighbor list,” of which pilot channels to monitor. If the pilot channel of a neighboring wireless coverage area is sufficiently stronger than the pilot channel of the current wireless coverage area, as measured by the mobile station, then a handoff of the mobile station from the current wireless coverage area to the neighboring wireless coverage area may be effected. In some cases, the handoff controller may be involved in effecting the handoff, e.g., if the mobile station is engaged in the call. When the handoff controller is involved in the process, the mobile station may report the measured signal strengths to the handoff controller, and the handoff controller may send a “handoff direction message” to the mobile station. In other cases, e.g., if the mobile station is an idle state, the mobile station may effect the handoff autonomously.
In a conventional approach, the mobile station measures the signal strengths of the pilot channels as relative signal levels, i.e., the ratio of the absolute signal level of the pilot channel to the total received power. Thus, in the example of a Code Division Multiple Access (CDMA) air interface, the relative signal level may be Ec/I0, where Ec is the energy per chip of the pilot channel and I0 is the total received power at that frequency.
However, it has been found that this conventional approach can cause difficulties. Problems can arise because noise and interference contribute to I0, such that I0 can be different in different types of locations. For example, in urban areas I0 can be anomalously high, due to many sources of noise and interference, thereby making Ec/I0 anomalously low. In rural areas, however, I0 can be anomalously low, due to few sources of noise and interference, thereby making Ec/I0 anomalously high. As a result of these differences, handoffs to wireless coverage areas in urban areas may occur less readily than may be desirable, and handoffs to wireless coverage areas in rural areas may occur more readily than may be desirable.